Penetrating dense objects at high velocity

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Discussion Overview

The discussion revolves around the counterintuitive phenomenon of dense objects, such as concrete, being penetrated by lighter materials, like wood, when moving at high velocities. Participants explore various scenarios, including comparisons to experiments from the Mythbusters show, and consider the implications of relative motion and kinetic energy in these interactions.

Discussion Character

  • Debate/contested
  • Exploratory
  • Technical explanation

Main Points Raised

  • Some participants find it counterintuitive that a moving concrete slab could be impaled by a stationary piece of wood, questioning the underlying physics.
  • Others argue that the kinetic energy of the moving object is crucial, noting that a piece of wood has less kinetic energy than a concrete slab moving at the same speed.
  • There is a discussion about the equivalence of motion, with references to Galileo's principles, suggesting that the outcomes should be the same regardless of which object is moving.
  • Some participants propose that pre-existing defects in the concrete could explain the penetration, likening it to the behavior observed in Mythbusters experiments with straw and palm trees.
  • Concerns are raised about the validity of the Mythbusters scenario, with some arguing that the mechanics of wood penetrating concrete differ significantly from wood penetrating softer materials.
  • Participants express skepticism about the specific case of concrete being penetrated by wood, suggesting alternative explanations and questioning the assumptions made in the original claims.

Areas of Agreement / Disagreement

Participants generally do not reach a consensus, with multiple competing views remaining regarding the mechanics of penetration and the role of relative motion. Some agree on the counterintuitive nature of the phenomenon, while others challenge the explanations provided.

Contextual Notes

Limitations include the dependence on specific conditions of the materials involved, the potential influence of pre-existing defects in the concrete, and unresolved questions about the nature of kinetic energy in different frames of reference.

  • #31
Few personal observations...
1. Pine needles can penetrate more than 0.50" into Chicago brick. Source: my house in 1979 hurricane Fredric.
2. Cedar is not stronger pine. Take a 2x4 of each and hit it on the endgrain with a 22oz framing hammer, cedar will crush and pine will have a dent(lodgepole) or barely a mark (southern yellow).
3. Concrete comes in different strengths from 1500psi to 7000psi mix. Curbs vary, but 2500-3500psi would be reasonably.

I agree with earlier comments about a drain hole or at least a flaw where the wood is.

PS. Never thought I'd have to "shave" my house. ;)
 
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  • #32
If you imagine suspending the wood by a piece of thread or fishing line or something and then whacking it at adequate speed with a concrete slab, it doesn't seem particularly counter-intuitive to think that the wood would be made to penetrate the rock. My intuition only has trouble when I imagine having the wood like resting on the ground or something and then hurling the rock at it. The trouble is that the wood in that case would always break before it had a chance to pierce the rock. But if you suspend it in such a way that there's nothing for the wood to break against, the penetration seems much more plausible.

So maybe the asymmetry is that the concrete is sturdy enough so as not to be destroyed by the impact, whereas the wood must be given special treatment so as not to be destroyed.
 
  • #33
Beach Geek said:
2. Cedar is not stronger pine. Take a 2x4 of each and hit it on the endgrain with a 22oz framing hammer, cedar will crush and pine will have a dent(lodgepole) or barely a mark (southern yellow).
It really depends on what you are talking about when you say "strength" and most importantly when you say "cedar", there is western red cedar and Canadian or Alaskan cypress which is called cedar. Pine has resin channels where as cedar doesn't, there for its harder because the grain is tighter.

http://woodmonsters.com/strength-chart.html
 
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